It is well known that a substantial portion of the hydrocarbon fractions used to prepare fuels such as motor gasoline are derived from the catalytic cracking of heavy petroleum fractions such as vacuum gas oils. The cracked materials are typically distilled into fractions including naphtha, heating oil, and diesel fuel.
Cracked naphthas are obtained from catalytic crackers in relatively high volumes and generally have good octane numbers. They are major components in blending for the motor gasoline pool. A substantial portion of the octane rating of cracked naphtha is due to its high olefin content.
Among the primary contaminants in feeds to fluidized catalytic cracking units (FCCU's) are sulfur bearing materials. These sulfur contaminants result in the appearance of sulfur species in the cracked products. Heavy gasoline components are known to accumulate sulfur impurities. It is well known to subject sulfur contaminated components to hydrodesulfurization to reduce the concentration of sulfur-bearing species in the desirable components.
Hydrotreating of petroleum feedstocks to remove heteroatoms, particularly sulfur, is an important factor in refineries in order to meet ever more demanding regulations relating to sulfur in fuels. These regulations are in response to environmental concerns, and the regulatory pressure to reduce the sulfur content of fuels has been, and will likely continue to be, directed at increasingly smaller concentrations.
Hydrodesulfurization is one of the fundamental hydrotreating processes of the refining and petrochemical industries. The removal of feed sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfides, especially those of Co/Mo and Ni/Mo, at fairly severe temperatures and pressures to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process such as reforming.
Some naphtha fractions containing olefins, for example, cracked naphthas or coker naphthas, typically contain over about 20 wt. % olefins. At least a portion of the olefins is hydrogenated to saturated hydrocarbons during the hydrodesulfurization operation. Because olefins are high octane components, it is usually desirable to retain the olefins rather than to convert them to saturated compounds. Conventional fresh hydrodesulfurization catalysts have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts under conventional startup procedures and under conditions required for sulfur removal, produces a significant loss of olefins through hydrogenation. This results in a lower grade fuel product which needs more refining, such as isomerization, blending, or other refining, to produce higher octane fuel, adding significantly to production expenses.
Selective hydrodesulfurization to remove sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts, have been described. For example, U.S. Pat. Nos. 4,132,632 and 4,140,626 disclose selective desulfurization of cracked naphtha using specific catalysts having particular amounts of Group VI and VIII metals on magnesia support. Also described is a process for starting-up naphtha hydrodesulfurizaton using partially deactivated hydrotreating catalyst under relatively low pressure in U.S. Pat. No. 4,149,965. The catalyst is partially deactivated using a substantially non-metals containing hydrocarbonaceous oil.
Hydrodesulfurization catalysts age, losing activity during use by collecting deposits of carbonaceous material and/or impurities, such as metals, from the treated feedstock. Eventually, with increased deposition, the catalyst is no longer able to provide effective hydrodesulfurization. The deactivated catalyst may be regenerated and reused, but is generally less effective than fresh catalyst by requiring higher temperature to give the desired activity and becoming deactivated more quickly than fresh catalyst. Although hydrodesulfurization catalysts can usually be repetitively regenerated, they eventually become irreversibly deactivated, or spent, essentially losing their intended hydrodesulfurization utility.
It is known to hydrodesulfurize an olefinic naphtha by adding a nitrogen compound to the feed in order to deactivate the catalyst for the hydrodesulfurization reaction, see U.S. Pat. No. 2,913,405.
Hydrodesulfurization of middle distillate, virgin oils using spent hydrotreating catalysts has been carried out under extremely mild conditions to reduce acid and mercaptan content, see U.S. Pat. No. 3,876,532. Also known in the art is a process for selectively hydrodesulfurizing naphtha by contacting the naphtha, which contains olefins and thiohydrocarbons, with hydrogen under vigorous hydrodesulfurization conditions in the presence of essentially deactivated hydrodesulfurization catalyst which selectively produces hydrogen sulfide, desulfurized hydrocarbons and a relatively high olefin content, see U.S. Pat. No. 5,286,373.
Finally, U.S. Pat. No. 6,197,718 B1 discloses a catalyst activation process in which the catalyst is heated in the presence of a virgin naphtha. The activated catalyst is used in the selective hydrodesulfurization of a naphtha containing sulfur and olefins. While this is effective, this process has drawbacks, such as, for example the cost of facilities to bring virgin naphtha into the cracked naphtha hydrotreater.
Although some of the above processes have met with commercial success, there still remains a need in the art for improved activation processes for cat naphtha desulfurization catalysts that do not require the use of an additional deactivation step or the use of a feed that would not normally be readily-available.